VARIATION IN A JKLLYl'LSH. 1049 



As mentioned before, variation in the number of radn is 

 of common occurrence in medusse, and in the above-described 

 varieties of Mcerisia lyonsi we are undoubtedly dealing with 

 a quite ordinary case of meristic variation. As Bateson remarks 

 in his well-known book on the Study of Variation [15] : — 

 " In radial series phenomena analogous to those of the variation 

 in linear series are seen in their simplest form. Just as in linear 

 series the number of meml)ers maybe changed by a, reconstitution 

 of the whole series so that it is impossible to })oint to any one 

 member as the one lost or added, so may it be in tlie meristic 

 variation of radial series : and again as in linear series, single 

 members of the series may divide. J^etween tliese there is no 

 clear line of distinction." 



In the abnormal specimens of Afa^risia, I do not think tliat we 

 are dealing with c;ises of division or suppression of one or more 

 radii ; from what we now know of the development of medusa-buds 

 [13, 14], it seems that the variation is in the number of radial 

 pouches formed in the early bud. 



Since each pouch develops the radial canal and perradial tentacle 

 of its own section of the medusa-bell, such variation explains tlie 

 correlation of tlie numbers of these two sets of organs. The fact 

 that the manubrium is independent of the oi'gans situated in the 

 vimbrella is a point decidedly in favour of this view, I have, 

 unfortunately, not been able to study the development of an 

 abnoi'mal specimen, all the medusa-buds 1 have sectioned pi'oved 

 to be of normal tetramei'ous symmetry. 



(6) The second gx'oup of abnormal individuals includes a 

 number of tetramerous medusae which bear secondary tentacles 

 between the four primary perradial ones. Tliese secondary 

 tentacles may be interradial, adradial or subradial in position, 

 and differ from the perradial ones in not communicating with the 

 gastric cavity by means of radial canals ; they are developed from 

 the ectodermal and endodermal cells of the umbrella-margin, their 

 cavities being in communication with the circular canal. 



Altogether 44 of the 400 tabulated individuals exhibited this 

 kind of abnormality, which is of a very peculiar type ; I have not 

 been able to find any record of a similar case in a medusa which 

 normally bears primary tentacles only. Thei'e are, of course, 

 numerous tetramerous medusse which noi'mally posssess. such 

 secondary tentacles (e. g., Podocoryne, l^arritoj^sis, Oceania, 

 Limnocnida), and they are known to start life with the four 

 primary ones only, the secondary tentacles being developed as 

 the animals grow, usually in some definite sequence. Thus, in 

 the majority of cases, the second set of tentacles to appear is the 

 interradial one, four of these structures growing simultaneously 

 from the umbi-ella-margin between the four perradial tentacles. 

 Eight adradial tentacles are formed next, two in each quadrant 

 occupying the interspaces between the interradial and perradial 

 ones, and their formation may be followed by the appearance of a 

 varying numV)er of subraflial tentacles. 



